Middle-aged and older adults represent a high risk group for obesity development. However, there is little known about the regulation of energy and nutrient balance, thus, about the mechanisms by which the obese state is achieved in middle-aged and older adults. Accordingly, the purpose of the present investigation is to determine: (1) the influence of age on the ability to adjust 24-hr energy expenditure (EE) and nutrient oxidation in response to short-term overfeeding; and (2) if age-related difference in the mass and metabolic profile of skeletal muscle are associated with the ability to adjust 24 hr EE and nutrient oxidation, respectively, in response to short-term overfeeding. My hypothesis are that (1) compared to young adults, older humans will demonstrate a smaller increase in 24-hr EE, a more rapid decline in the fat-to-carbohydrate oxidation ratio, and a more positive fat balance in response to short-term overfeeding; and (2) skeletal muscle mass will be closely associated will the ability to adjust 24-hr EE and the (beta)- hydroxyacyl CoA dehydrogenase (beta)-HAD/citrate synthase (CS) [indicative of the capacity for fat oxidation], phosphofructokinase (PFK)/(beta)-HAD, and PFK/CS [indicative of the capacity for CHO oxidation] ratio in skeletal muscle will be closely related to the fat- to-CHO oxidation ration and nutrient balances in responses to short-term overfeeding. TO address this issue, 8 young (age=18-30 yrs) and 8 middle-aged and older adults (age=55-65 yrs) will be fed a mixed diet 150% above baseline energy requirements for 7 days. Measurements of 24- hr EE and nutrient oxidation, via whole room calorimetry, will be made on days 0, 1, and 7 of overfeeding. The mass (dual x-ray absorptiometry) and metabolic profile (muscle biopsies of the vastus lateralis) of skeletal muscle will be obtained at baseline in order to examine the relationships to the adjustments in 24-hr EE, the fat-to carbohydrate oxidation ratio, and the oxidation rate (and balance) of each nutrient (fat, carbohydrate, and protein). The results of the present study should provide new and clinically important information regarding the influence of age on the regulation of energy and nutrient balance in humans. In addition, new and important information will be provided regarding the metabolic consequences of age-related changes in the mass and metabolic profile of skeletal muscle. Understanding the mechanisms by which middle-aged and older adults gain total body fat and achieve the obese state will lead to improved strategies for the prevention and treatment of obesity and related health problems in this population.